Relation of Nucleoside Transport and Surface Phosphohydrolase Activity in Hymenolepis diminuta

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The influx of 4C-uridine into Hymenolepis diminuta was saturable, and inhibited competitively by adenosine, AMP, and ATP. It was postulated that hydrolysis of the latter 2 compounds by a tegumentary phosphohydrolase, also acting in p-nitrophenylphosphate (PN4P), yields products responsible for inhibition of uridine influx. Hydrolysis of PN4P was inhibited competitively by AMP and ATP, and the inhibition of uridine influx by ATP was reversed by PNDP, molybdate, NaF, and fructose diphosphate. Inhibiton of uridine influx by AMP was not reversed by these latter reagents. Glucose-l-phosphate (C1P) and glucose-6-phosphate (G6P) did not relieve the AMP or ATP inhibition of uridine influx. Apparent influxes of G1P or G6P were inhibited completely by AMP and glucose, and inhibited partially by ATP. These observations are consistent with previous reports that hexose phosphate hydrolysis precedes absorption. Adult tapeworms are an exceptional material for the study of membrane transport in an intact organism. The external surface is comprised of a syncytial epithelium with numerous microvillar projections, the surface ultrastructure resembling that of the intestinal mucosa in many other animals (Lumsden, 1966). Since tapeworms lack a digestive tract, absorption of nutrients from the medium must occur across this tegument. Maclnnis and Litchford (1965) and MacInnis and Ridley (1969) reported that the tapeworm Hymenolepis diminuta has a mediated system for nucleoside transport. These authors found nucleoside transport to be inhibited by nucleosides but not by purine or pyrimidine bases. Using ultrastructural cytochemical methods, Lumsden et al. (1968) showed that nucleotides are hydrolyzed at the brush border and suggested that nucleoside transport might be inhibited by liberated nucleosides. Arme and Read (1970) and Dike and Read (1971a) showed by both chemical and ultrastructural methods that the hydrolysis of fructose phosphate esters occurs at the tegumentary surface of H. diminuta. As hydrolysis Received for publication 10 December 1973. *This study was completed while the senior author was a NIH Postdoctoral Fellow (5-F02AI-45108-02) at Rice University, and was supported in part by a grant (AI-01384) from the NIH. t Deceased. proceeds free fructose accumulates in the external medium, since the worm is essentially impermeable to fructose. In further studies, Dike and Read (1971b) showed that the apparent absorption of glucose phosphate esters is preceded by hydrolysis of the esters; the phosphohydrolase and glucose transport systems can be differentially inhibited. The latter authors showed that the spatial arrangement of the glucose transport system and the phosphohydrolase results in a kinetic advantage for the absorption of glucose liberated by hydrolysis of glucose phosphate esters. That is, the liberated glucose is absorbed, rather than diffusing into the medium. The present paper reports some experiments designed to determine whether nucleotides affect nucleoside transport, and to test the hypothesis that nucleotide hydrolysis may precede inhibition of nucleoside transport by nucleotides in H. diminuta. MATERIALS AND METHODS Hymenolepis diminuta was maintained in the beetle, Tenebrio molitor, and male SpragueDawley rats (Holtzman Co., Madison, Wisconsin). Rats weighing 60 to 80 g were infected with 30 H. diminuta cysticercoids and worms were recovered 10 days postinfection. Worms from several rats were pooled and rinsed in several changes of a Krebs-Ringer saline containing 25 mM tris (hydroxymethyl) aminomethane-maleate buffer at pH 7.4 (KRT of Read et al., 1963), and randomized into groups of 5. Each group was preincubated in 10 ml of KRT at 37 C for 15 min and then transferred to 5 ml of KRT (at 37 C) containing substrate and inhibitors at the appropriate